Epoxides in Titanocene‐Mediated and ‐Catalyzed Radical Reactions

Abstract

Abstract Epoxides are among the most useful functional groups in organic chemistry. They are also excellent radical precursors. Ring opening can be achieved via electron transfer from low valent metal complexes. As yet, titanocene(III) chlorides have emerged as the most useful reagents in this respect. Catalytic reaction conditions have been described that exploit the stability of radicals under protic conditions. The use of enantiomerically pure titanocenes has resulted in a number of applications in enantioselective catalysis. Most recently, this has been demonstrated in enantioselective cyclizations. Other unexpected developments have been reported in the field of hydrogen atom transfer reagents for the reduction of radicals generated from epoxides. Titanocene bound water acts as an excellent hydrogen atom transfer (HAT) reagent. Moreover, H 2 can be employed as terminal reductant when activated by “hydrogenation” catalysts, such as RhCl(PPh 3 ) 3 . The most common application of the titanocene‐mediated or ‐catalyzed epoxide opening is in cyclization reactions. It has been demonstrated that cyclopropanes, cyclobutanes, cyclopentanes, cyclohexanes, and cycloheptanes are readily accessible. It is expected that these cyclizations will be increasingly frequently applied in the synthesis of natural products.